This invention relates, in the broadest sense, to the fabrication of integrated circuit dice (chips), and, more specifically, to an improvement in the metallization technique for forming conductors on said dice so as to improve the conductor, or die attachment surface wettability (bondability) to other conductors or surfaces to form reliable physical or electrical connections therebetween.
An example of the application of this invention will be described in connection with the attachment of a silicon die to a bonding pad of a ceramic package in which the die is to be contained, although other applications of this invention will become apparent to those skilled in the art from the following description.
Integrated circuit dice are presently packaged, in many instances, in ceramic type packages which have external leads to connect the packages to other circuitry. Each die is physically bonded to the package either by an epoxy or by metallurgical bonding to form an electrical or physical contact with the die. A present method of making this electrical or physical contact is to deposit a thin film of Au, or Cr then Au, on one side of the die. Usually, the bonding pad of the ceramic package has an area of low melting Au paste and, this low melting Au paste, or Au alloy, such as Au--Si or Au--Ge, is sandwiched between the bond pad and the backside of the die and the two are subjected to a heat cycle plus ultrasonic energy to firmly attach the die to the pad.
Presently this prior art technique has been found deficient because of premature separation of the die and from the bonding pad which results in a functional failure of the integrated circuit chip. In other cases, the bond, being sufficiently weak, will result in a separation between the die and the pad when the package is subjected to shock, etc. The cause of the low bond strength is often found to be poor wettability (bondability) of the conductor material (usually Au) on the die. An Auger depth profile analysis of this metallization often shows considerable SiO.sub.2 on the surface of the Au especially after the heat treatment step for attaching the die to the bonding pad. It appears that the Si diffuses through the Au where oxidation takes place, thus reducing the wettability of the Au. It has been observed that this oxide can grow at about 1000 .ANG. per minute at 375.degree. C.
This means that a continuous oxide non-wettable surface can form in only a few seconds at 375.degree. C. To reduce the rate of this Si diffusion, a Cr layer has been employed as a barrier layer between the Si and the Au; however, it has been observed that movement of the Cr through the Au, in addition to the Si movement, occurs after about 3 minutes at 375.degree. which still results in a non-wettable surface.
Another negative aspect of the prior art process is the need for larger amounts of Au to be used due to this diffusion of the Si to the surface of the Au where the oxidation takes place. It is almost as if large amounts of Au disappear into the barrier layer and Si layer.
It is, therefore, a prinicipal object of this invention to improve the properties of integrated circuits by increasing the wettability of conductive material in the process of manufacturing these circuits.
Another specific object of this invention is to provide a method of manufacture of integrated circuits which improves the strength and reliability of the die attachment metallization technique.
Still another and more specific object of this invention is to obtain a reduction in Au usage in the die attachment metallization technique and permit other less expensive bondable materials such as silver, or silver alloys to be used.